Compensating pressure tank for oil-filled power cables

ABSTRACT

A tank contains a first chamber having a bellows-type elastic wall and has a normally open valve through which the chamber is connected to an oil-filled cable and when the chamber wall contracts sufficiently the valve is engaged by the wall and closed. The first chamber is surrounded by a fluid filled second chamber which is connected to a source of fluid through a similar valve which is normally open but which is closed by contact with a wall of the first chamber when the wall thereof expands by a predetermined amount.

United States Patent Ferrentino COMPENSATING PRESSURE TANK FOROIL-FILLED POWER CABLES [72] Inventor: Antonio Ferrentino, Monza, Italy[73] Assignee: Industrie Pirelli Societa per Azioni,

Milan, Italy [22] Filed: May 24, 1971 [21] Appl. No.: 146,295

[30] Foreign Application Priority Data May 23, 1970 Italy ..25005 A/70[52] U.S. Cl. ..l38/30 [51] Int. Cl ..F16l 55/04 [58] Field of Search138/26, 30

[56] References Cited UNITED STATES PATENTS 7/1969 Legrand ..138/30 [451Oct. 3, 1972 Primary Examiner-Henry T. Klinksiek Attomey-Brooks, Haidt &Haffner [57] ABSTRACT A tank contains a first chamber having abellows-type elastic wall and has a normally open valve through whichthe chamber is connected to an oil-filled cable and when the chamberwall contracts sufficiently the valve is engaged by the wall and closed.The first chamber is surrounded by a fluid filled second chamber whichis connected to a source of fluid through a similar valve which isnormally open but which is closed by contact with a wall of the firstchamber when the wall thereof expands by a predetermined amount.

13 Claims, 2 Drawing figures PATENTEDucra m2 SHEET 2 [1F 2 I IN VENTOR.flwromo @Pcwrwo 147 Tole/V678 COMPENSATING PRESSURE TANK FOR OIL- FILLEDPOWER CABLES The object of the present invention is to provide apressure tank for the compensation of the volume of fluid oil,especially suited for installation alone, or in parallel with otherpressure tanks of the same type, on power cables subject to very highpressures.

The pressure tanks for the compensation of the variations in the volumeof oil, due to the variations in the temperature of the fluid oil inpower cables, have, up till now, been constructed with groups oflenticular elastic cells, arranged in parallel, each communicating withthe other, filled with gas at atmospheric pressure or .preloaded, andimmersed in a container filled with the cable oil and therefore,sensitive on their surface to the variations in the oil pressure.

In other types, the cells are filled with oil and immersed in gas or inoil on which a suitable pressure is exerted.

These cells, which serve the double purpose of being elastic separatorsbetween fluids of the same or different natures and elements for thetransmission of the pressure exerted by the containing or containedfluid, are limited in their application in that they give good resultsso long as the pressure does not exceed levels of about 4 atmospheres.With the improvement in cable techniques and methods of production, thetendency is to make cables, with the same electrical characteristics andtherefore the same transversal dimensions in all of their parts, longerand longer, especially for underwater installations. It follows,therefore, that in order to ensure the presence of insulating oil inevery point of the cable, even higher pressures must be reverted to,sometimes reaching or exceeding atmospheres.

Under these conditions, the use of pressure tanks with lenticular cellsbecomes critical since beyond a certain limit of pressure there occur inthe cell walls, which are subject to fatigue stress, some localizeddeformations, which exceed the elasticity limit permitted by thematerial of which they are composed. Such deformations cause radialridges to form which may lead to a break. It is well known that a breakin one cell not only endangers the life of the pressure tank but alsoleads, in most cases, to the contamination of the cable insulating oilwith serious prejudice to its dielectric properties. Moreover, thetypical curve which, plotted on a pair of cartesian axes, indicates thevaria tion in the cell volume along the abscissa in accordance with thevariation in pressure applied to it along the or dinate is rather slopedwith respect to the horizontal and this slope increases with theincrease in pressure. This means that for the desired variations involume corresponding considerable variations of pressure are required,and therefore, due to the above behavior of the cells at high pressure,it is understandable that their use is limited to modest volumevariations.

The purpose of the present invention is to overcome the abovedisadvantages, and to provide a pressure tank especially suited to thehigh pressure and which will provide a typical pressure-volume curvewhich is sufficiently flat to produce, with small pressure variations,considerable displacements of volumes.

The present invention provides, moreover, the adthe important quality ofmeeting high pressure requirements, can, in its overall formulation, bedesigned for pressures of any value, making the elastic separator havethe sole function of separating, without subjecting it to differences inpressure between its inner and outer surfaces and hence, prolonging itsfatigue resistance to the maximum irrespective of its mechanicalstrength.

More precisely, the object of the present invention is a compensationpressure tank for oil-filled power cables in which one single outercover contains at least a first and a second chamber, the first chamber,having an elastic wall, being filled with the degasified oil of thecable communicating with it and being immersed in the second chamber andsealed off from it, this second chamber being filled with a liquid,preferably oil and being in communication with a third expansionchamber, such tank being characterized by the fact that the firstchamber has a fixed and rigid base plate, an elastic, bellows-typeperipheral surface, and a mobile rigid closing plate, the communicationbetween the first chamber and the cable being achieved by means of atleast a first stop-valve fitted on the base plate in line and in mirrorrelationship with respect to a second stop-valve fitted on an upperplate of the second chamber. The first and the. second stop-valves, intheir normal positions, are at a distance from said mobile plate and arein positions such as to form, with the plate, a hydraulic cut-off andare operated by the same alternately with compression or expansion ofthe first chamber.

The attached drawings show, as examples, two preferred embodiments ofthe invention and in such drawings:

FIG. 1 is a longitudinal cross-section of a compensation pressure tankof the invention with the second expansion chamber contained in a singleunit and a gas supply tank; and

FIG. 2 shows a longitudinal cross-section of a compensation pressuretank of the invention with the second expansion chamber as a separateunit.

The compensation pressure tank in FIG. 1 includes a tank enclosing avolume divided into a first elastic chamber A, a second chamber B and athird expansion chamber C.

The first elastic chamber A is mainly within the second chamber B and isdefined by a fixed and rigid base plate 6, .coinciding with the base ofthe tank 1, a wall 2 of stainless steel having a bellows-type peripheralsurface which acts as an elastic membrane, a flange 4 welded to the wall2, the flange 4 engaging sealing washers and being held in fluid-tightrelation between the hollow cylindrical wall 5 of the outer cover 1 andthe fixed base plate 6, and a movable rigid closing plate 3, preferablyof stainless steel, secured to the upper end of the membrane 2.

The first elastic chamber A, which is sealed off from the second chamberB, contains degasified insulating fluid oil and communicates with thecable (not shown) through a stop-valve 7, inserted in the center of thefixed base plate 6. The valve 7 comprises a hollow cylindrical body 8,subdivided into a first cavity 10 and a second cavity 11.

The first cavity 10 has an end opening 9 for providing a connection withthe cable and contains, coaxially, a first helical spring 12 which restson the surface surrounding the'opening 9 and is connected at the otherend with a first shutter 13 fitted with a ring washer 23 which forms afluid seal when in contact with seat 14 of said first shutter 13.

The shutter 13 is axially movable in the second cavity 11 which containssecond helical spring 15, stiffer than the first spring 12 and restingat one end on said shutter valve 13, while the other, or free end fitsin a piston 16 joined by a small shaft to a second shutter 17 outsidethe valve housing and having on its own inner surface a seal or ringwasher 18. The shutter 17 is made to fit into a corresponding seat 20formed at the free edge 19 of the valve housing. The piston 16 has aplurality of channels 28 at the end thereof, which permit oil to flowpast the piston 16 when it is against the stop ring 29.

The second chamber B is full of a liquid, preferably oil, and isseparated from the expansion chamber C, by a top plate 22. A secondstop-valve 7, fitted in the center of plate 22, puts the second chamberB in communication with the expansion chamber C.

The stop-valve 7', the corresponding parts of which are marked with thesame reference numbers, but primed, as the corresponding parts of thefirst stopvalve 7, is the same, in line with and exactly mirrors thefirst valve 7. In their normal positions, valves 7 and 7 are at adistance from the movable plate 3 and in a position such as to form withthe plate 3 a hydraulic cut-off. The valves 7 and 7 are operatedalternately by the plate 3 due to the compression or expansion of theelastic chamber A. The expansion chamber C is partially filled with oiland has a greater volume than the oil flowing into it for maximummigration of oil from the cable to the elastic chamber A. The remainingspace in chamber C is filled with a gas under pressure such as air,nitrogen, etc. The oil contained in the chamber C is, in volume, such asto guarantee its presence, whatever the displacement of oil from theChamber A to the cable.

In the preferred embodiment of FIG. 2, the expansion chamber C is a tankseparate from that containing the other two chambers A and B, andcommunicates with the chamber B through an appropriate pipe 27,connected to the second stop-valve 7. The tank containing chamber C may,therefore, be placed at any distance from that enclosing the chambers Aand B.

The gas under pressure for the chamber C is supplied from an appropriatetank D (shown in FIG. l-only) separate from the tank 1, although it maybe alongside or on top of the expansion chamber C or it may be combinedwith said expansion chamber C as shown in FIG. 2.

The tank D of FIG. 1 or the combined tank containing the chamber C asshown in FIG. 2, which could moreover be only one or several pressuretanks arranged in parallel, is also fitted with a warning pressureswitch 24 which signals insufficient pressure, with a safety valve 25and a filling cock 26. The tank D has dimensions proportioned so as torestrict the variations of gas pressures within the limits assigned tothe oil pressure, and it is preloaded when it is wished to keep the oilpressure always above a certain value.

The oil which, during the working of the cable, enters from the cableinto chamber A or, vice versa, travels from chamber A to the cable,lengthens or, respectively, shortens the elastic bellows-type membrane 2and displaces an equal volume of oil from the chamber B to the chamber Cor vice versa. The stroke of the elastic bellows-type membrane or itselongations and shortenings are limited hydraulically by the valves 7and 7', operated alternately by the movable plate 3, such valvespreventing the bellows-type walls of the membrane 2 from sufieringharmful and substantial differences of pressure between the inside andthe outside so that the elastic membrane 2 operates exclusively as aseparator between the two volumes of oil in the chambers A (degasifiedinsulating oil) and B (oil in contact with the gas of chamber C).

If, for example, due to a cooling of the cable, the degasified oilshould flow from the chamber A towards the cable, the elasticbellows-type membrane 2 will contract until it contacts, by means of themovable plate 3, the shutter 17 of the valve 7, the piston 16 of whichwill slide downwards and by means of the spring 15, stifi'er than thespring 12, will push, in turn, the shutter 13 to rest, with the ringwasher 23, on the cor responding seat 14, causing the closure of thevalve 7. As can be seen, the spring 15 makes it possible to close theshutter 13 with a pre-established force and not with all the force whichthe plate 3 is capable of exerting under differences in pressure whichmay even be very slight. From this moment no more oil may leave thechamber A and in the chambers A and B the liquids are at the samepressure, excluding in this consideration the negligible hydrostaticpressures and the slight negligible difference in pressure between theelastic chamber A and the second chamber B caused by the weak reactionof the bellows-type surface 2 when it is not in its rest position.

If the washer 23 did not ensure a perfect seal, there would be apersistent flow of oil towards the cable which would make the movableplate 3 descend still further due to compression of the spring 15, untilthe shutter 17 touches, with its washer 18, the corresponding seat 20.This second closure constitutes a guarantee against the possiblenon-operation of shutter 13.

If, instead, the cable were to heat up, and consequently the oilincrease in volume and tend to migrate from the cable to the chamber A,the plate 3 would rise, moving the oil from the second chamber B to theexpansion chamber C, opposing the pressure of the gas above the oil,through the stop-valve 7 until, coming in contact with the shutter 17 ofthe stopvalve 7', it would shut the latter. The operation of the stopvalve 7' corresponds to that previously described for the stop-valve 7.

Due to the action of the plate 3 on the shutter 17 the piston 16 pushesup the spring 15 which, being stiffer than the spring 12, forces thelatter to compress, putting the shutter 13' with its associated washer23' into contact with the seat 14', so achieving a first closure of thestopvalve 7 If however the oil continues to flow, due to a poor sea] bythe washer 23', towards the expansion chamber C, a further slightdisplacement of the mobile plate 3 puts the shutter 17' with itsrespective washer 18' into contact with the seat 20 formed at the edge19.

The stop-valves 7 and 7 with their function as hydraulic cut-offs forthe movable plate 3 which prevent the elastic bellows-type surface 2from being subjected to differences in pressure on the outside and theinside, thereby increasing the fatigue resistance to the maximum.

The constructional details of the invention may naturally be variedaccording to requirements. For example the cable communication valvesand the valves communicating with the expansion chamber C may be morethan one, or each stop-valve may comprise more than two cavities inseries.

I claim:

1. A compensating pressure tank for oil-filled power cables, said tankcomprising an elastic wall dividing at least a portion of said tank intofirst and second closed chambers, a first normally open valvecommunicating with said first chamber and having an actuating member andmeans extending through a wall of said tank for connecting an oil-filledpower cable thereto and permitting the oil in said cable to flow intoand out of said first chamber, said valve being positioned with itsactuating member engagable by a portion of said wall upon movement ofsaid wall in a first direction corresponding to a decrease in the oilpressure in said cable to close said valve and a second normally openvalve communicating with said second chamber and having an actuatingmember and means extending externally of said second chamber forconnecting a source of fluid thereto and permitting the flow of fluidinto and out of said second chamber, said second valve being positionedwith its actuating member engagable by a portion of said wall uponmovement of said wall in a second direction corresponding to an increasein the oil pressure in said cable to close said second yalve.

2. A tank set forth in claim 1 wherein said source of fluid is anexpansion chamber means having a gas therein for applying pressure tofluid therein.

3. A tank as set forth in claim 2 wherein said expansion chamber meansis part of said tank.

4. A tank as set forth in claim 2 wherein said expansion chamber meansis separate from said tank.

5. A tank as set forth in claim 2 wherein the fluid volume of saidexpansion chamber means is at least as great as the volume change withinsaid first chamber as said portion of said wall moves from a positionadjacent to the actuating member of said first valve to the position inwhich it closes said second valve.

6. A tank as set forth in claim 1 wherein said elastic wall is a bellowsmounted at one end on a wall of said tank and extending therefrom intosaid tank.

7. A tank as set forth in claim 6 wherein said wall of said tank isrigid and the other end of said bellows remote from said tank wall has arigid place secured thereto.

8. A tank as set forth in claim 7 wherein at least one of said bellowsand said rigid plate is made of stainless steel.

9. A tank as set forth in claim 7 wherein said first valve is mountedwithin said bellows with its actuating member adjacent to said rigidplate and engagable by the portion of said bellows to which said rigidplate is secured.

10. A tank as set forth in claim 9 wherein said second valve is mountedon a wall of said tank external to said bellows with its actuatingmember adjacent and engagable by said rigid plate.

11. A tank as set forth in claim 1 wherein each of said first and secondvalves comprises a hollow cylindrical body having an opening at one end,a seat internally of said body facing away from said one end, said seatbein intermedi te s aced from said one end and the ther end 0 sai d fioy, .a shutter mounted within said body and intermediate said seat andsaid other end of said body, said shutter being engagable with saidseat, a piston mounted intermediate said shutter and said other end ofsaid body, a first spring mounted intermediate said one end of said bodyand said shutter, and acting between said shutter and said body to urgesaid shutter away from said seat and normally maintaining it spacedtherefrom, a second spring, stiffer than said first spring, mountedintermediate said shutter and said piston and acting therebetween tourge said shutter toward said seat and said piston toward said other endof said body and stop means for limiting the movement of said piston.

12. A tank as set forth in claim 11 wherein said body has an externallyfacing seat at said other end thereof and said piston carries a furthershutter engagable with said externally facing seat but spaced therefromwhen said piston is in engagement with said stop means said piston andsaid further shutter forming said actuating member for the valve.

13. A tank as set forth in claim 12 wherein said first and second valvesare mounted co-axially and with their open ends facing oppositely.

1. A compensating pressure tank for oil-filled power cables, said tankcomprising an elastic wall dividing at least a portion of said tank intofirst and second closed chambers, a first normally open valvecommunicating with said first chamber and having an actuating member andmeans extending through a wall of said tank for connecting an oil-filledpower cable thereto and permitting the oil in said cable to flow intoand out of said first chamber, said valve being positioned with itsactuating member engagable by a portion of said wall upon movement ofsaid wall in a first direction corresponding to a decrease in the oilpressure in said cable to close said valve and a second normally openvalve communicating with said second chamber and having an actuatingmember and means extending externally of said second chamber forconnecting a source of fluid thereto and permitting the flow of fluidinto and out of said second chamber, said second valve being positionedwith its actuating member engagable by a portion of said wall uponmovement of said wall in a second direction corresponding to an increasein the oil pressure in said cable to close said secOnd valve.
 2. A tankset forth in claim 1 wherein said source of fluid is an expansionchamber means having a gas therein for applying pressure to fluidtherein.
 3. A tank as set forth in claim 2 wherein said expansionchamber means is part of said tank.
 4. A tank as set forth in claim 2wherein said expansion chamber means is separate from said tank.
 5. Atank as set forth in claim 2 wherein the fluid volume of said expansionchamber means is at least as great as the volume change within saidfirst chamber as said portion of said wall moves from a positionadjacent to the actuating member of said first valve to the position inwhich it closes said second valve.
 6. A tank as set forth in claim 1wherein said elastic wall is a bellows mounted at one end on a wall ofsaid tank and extending therefrom into said tank.
 7. A tank as set forthin claim 6 wherein said wall of said tank is rigid and the other end ofsaid bellows remote from said tank wall has a rigid place securedthereto.
 8. A tank as set forth in claim 7 wherein at least one of saidbellows and said rigid plate is made of stainless steel.
 9. A tank asset forth in claim 7 wherein said first valve is mounted within saidbellows with its actuating member adjacent to said rigid plate andengagable by the portion of said bellows to which said rigid plate issecured.
 10. A tank as set forth in claim 9 wherein said second valve ismounted on a wall of said tank external to said bellows with itsactuating member adjacent and engagable by said rigid plate.
 11. A tankas set forth in claim 1 wherein each of said first and second valvescomprises a hollow cylindrical body having an opening at one end, a seatinternally of said body facing away from said one end, said seat beingintermediate and spaced from said one end and the other end of saidbody, a shutter mounted within said body and intermediate said seat andsaid other end of said body, said shutter being engagable with saidseat, a piston mounted intermediate said shutter and said other end ofsaid body, a first spring mounted intermediate said one end of said bodyand said shutter, and acting between said shutter and said body to urgesaid shutter away from said seat and normally maintaining it spacedtherefrom, a second spring, stiffer than said first spring, mountedintermediate said shutter and said piston and acting therebetween tourge said shutter toward said seat and said piston toward said other endof said body and stop means for limiting the movement of said piston.12. A tank as set forth in claim 11 wherein said body has an externallyfacing seat at said other end thereof and said piston carries a furthershutter engagable with said externally facing seat but spaced therefromwhen said piston is in engagement with said stop means said piston andsaid further shutter forming said actuating member for the valve.
 13. Atank as set forth in claim 12 wherein said first and second valves aremounted co-axially and with their open ends facing oppositely.